The present invention relates to a fluoroelastomer composition suitable for the production of electrical insulating materials. More particularly, it relates to a fluoroelastomer composition comprising a blend of two or more fluororesins. Also, the present invention is concerned with a heat shrinkable articles such as heat shrinkable tubes comprising a fluoroelastomer composition.
Generally, fluoroelastomers are highly resistant to heat, oil and chemicals and therefore they are used for various purposes such as for the production of gaskets, packings and hoses. In view of the characteristics of fluoroelastomers there has been increasing desire to use them as an electrical insulating material such as a covering material for electrical wires and heat shrinkable tubes for protecting terminals of communication cables and electric cables.
However, conventional fluoroelastomers have various defects that they are poor in electrical characteristics and mechanical strength, and therefore improvement has been desired.
Recently, tetrafluoroethylene-propylene based copolymers have been developed as a fluoroelastomer which has an excellent electrical characteristic. Such tetrafluoroethylene-propylene based copolymers include copolymers of (i) tetrafluoroethylene and (ii) propylene as major component monomers and (iii) one or more third component monomers copolymerizable therewith such as, for example, ethylene, isobutylene, acrylic acid, alkyl acrylates, vinyl fluoride, vinylidene fluoride, hexafluoropropene, chloroethyl vinyl ether, chlorotrifluoroethylene, and perfluoroalkyl vinyl ether in a suitable amount.
However, when the conventional tetrafluoroethylene-propylene copolymers are intended to be used as an electrical insulating material for electric wires, in particular as heat-shrinkable tubes they are unsatisfactory since they show a poor mechanical strength, and they are not strong enough after being extrusion-coated on wires to render the covered wires self-supporting and therefore they cannot be used as is.
Further, when the conventional tetrafluoroethylene-propylene copolymers are used for the production of heat-shrinkable tubes, the resulting tubes are poor in heat settability, that is, they are difficult to maintain their shape or form after expansion.
In order to improve the mechanical strength of fluororesins, it has heretofore been proposed to blend a fluoroelastomer with a fluororesin, in particular a crystalline polymer, such as ethylene-tetrafluoroethylene copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, polyvinylidene fluoride, vinylidene fluoride-fluoroolefin copolymers and the like as described in, e.g., U.S. Pat. Nos. 4,094,949 and 4,575,537.
However, by blending these fluororesins the low temperature characteristics of the fluoroelastomer composition is deteriorated and articles comprising such blends does not pass low temperature test at -10.degree. C. which ordinary insulated wires are required to pass although the mechanical strength of them is improved, and as a result they cannot be used practically.